A new study published in the scientific journal Addiction has found that the legalization of recreational cannabis in U.S. states appears to have caused a 20% average increase in cannabis use frequency in those states.

The study evaluated the effects of recreational cannabis legalization in a large sample of adult identical twins. Of particular interest were the 111 identical twin pairs in which one twin lived in a state with a different recreational cannabis policy to the other. Twins provide extremely well-matched controls for each other and permit more precise estimation of the causal impact of recreational legalization than studies of unrelated individuals.

Looking at all of the study participants (1,425 individuals living in states with legal recreational cannabis use and 1,997 living in states in which recreational cannabis use is illegal), the study found a ~24% increase in mean cannabis use frequency attributable to legalization. Looking just at the identical twins living in states with different policies, the twin living in a ‘legal’ state used cannabis ~20% more frequently than their cotwin living in an ‘illegal’ state. These findings suggest that recreational legalization caused an increase in cannabis use.

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